1. Field of the Invention
The present invention concerns x-ray detectors based on indirect x-ray conversion.
2. Description of the Prior Art
In the field of x-ray detectors, there is a fundamental differentiation as to whether the x-ray detector is based on direct or indirect x-ray conversion. These forms of x-ray detection are shown in FIGS. 1 and 2, respectively and described below in detail.
In direct x-ray conversion, the x-ray radiation is absorbed in a material and an electron-hole pair is generated with the energy of said x-ray radiation. The generated electron-hole pair can be read out electronically. Amorphous selenium is used as a material for this purpose, for example. Silicon diodes are also used for direct x-ray conversion. Direct x-ray conversion in a semiconductor depends on a certain layer thickness in order to absorb a sufficiently high proportion of the radiation for a detection. Silicon diodes for direct x-ray conversion have component thicknesses of approximately 1 cm. Layers of up to 1 mm in thickness are used for direct x-ray conversion in amorphous selenium. Selenium as an absorber is particularly disadvantageous due to its high toxicity.
For indirect x-ray conversion, it is known to use combinations of a scintillator layer and a photodetector. The spectral sensitivity of the photodetector is thereby in the wavelength range of the fluorescence emission of the scintillator layer that is generated by x-ray conversion. The scintillator layers are materials such as, for example, cesium iodide or gadolinium sulfur oxide. Since scintillators made from cesium iodide are extremely hygroscopic, their use in combination with photodetectors is always linked with a structural cost (for example for a moisture protection encapsulation) and is disadvantageous to the service life of the x-ray detector.
In addition to the two basic x-ray conversion forms that are possible, in the field of x-ray detection differentiation must also be made according to the field of use. In the field of use of x-ray imaging—for example the medical field—cost-effective and large-scale solutions are sought. This depends on a high spatial resolution for the imaging. Direct x-ray converters have previously been used for medical imaging, but those have significant component depths and therefore require high, energetically inefficient operating voltages in the kV range. Alternatively, a scintillator layer on a photodetector array is used which must disadvantageously be constructed from a number of pixelated photodetectors in order to ensure the desired spatial resolution.
In addition, x-ray detectors are used in x-ray dose rate measurement. In x-ray apparatuses in the medical field and in industry and safety technology, the x-ray dose is measured with components known as dosimeters. An efficient transduction of the absorbed x-ray radiation into a usable signal is important for the x-ray dose measurement. The signals should be sufficiently high and noise-free in order to determine a precise x-ray dose. A local absorption of the x-ray radiation is important for the use of the x-ray dose monitoring devices in combination with an x-ray imaging. This means that a clear signal must be generated from only a small amount of absorbed x-ray radiation. This is necessary in order to not generate any shadows on the x-ray image. Low absorption is necessary, however, in the dose measurement in order to keep the x-ray exposure (for a patient, for example) as low as possible for a clear x-ray image. Ionization chambers or thick silicon photodiodes have previously been used for monitoring of the x-ray dose rate, for example.
A disadvantage of known x-ray detectors is that they are suitable only for severely limited range of use. The different modes of operation for x-ray detection cannot be advantageously combined with one another. Moreover, all known x-ray detectors exhibit significant component depths.